We have examined the control of gene expression in Simian virus 40 (SV40). There are two mRNAs transcribed from the early region both of which arise from a common precursor. The relative proportion of the RNAs is not influenced by a mutation in large T-antigen, the SV40 specific protein which controls the transcription of these RNAs. However, the temperature of growth does alter the proportions of the two RNAs. This apparently occurs by shifting the splicing of the precursor RNA in favor of one of the RNAs. We believe that this shift indicates a type of control of gene expression which involves the stability of structures formed in the RNA template which are required for splicing. With changes in metabolic environment (i.e. temperature, pH, or ionic strength) the stability of a splicing structure would change, thus a precursor RNA, able to be spliced in several ways, may be shifted toward splicing of one RNA which codes for a specific protein needed by the cell (or virus) under a particular growth condition. Experiments to confirm this model of gene expression are in progress. In other experiments, we have studied the control of late transcription of SV40. These studies indicate a continual requirement of the SV40 early protein, large Tantigen, for late transcription. However, a host cell factor is probably involved as well and the interaction of this factor and large T-antigen varies between cell lines. Also studied is the synthesis of SV40 RNA in transformed cells.